Fix : coherency in const parameters

Fix : coherency in const parameters

This patchs adds the const attribute to the field access functions
(get_int64 and others) in order to keep the coherency with the get_field
and get_field_list function which return a const pointer.

fixes #84

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>